With the stroke of a pen, the U.S. Justice Department has just subjected hundreds of thousands of Americans to a lifetime gun ban.

In an open letter to all licensed firearms dealers, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) unilaterally decided that any person who is prescribed marijuana for medical purposes is prohibited from possessing a firearm.

That means over 300,000 people in the states that allow medical marijuana suddenly cannot buy or possess a gun. Oh yeah, and for the many who already own firearms, watch out. They are now felons and could be subject to lengthy prison terms of up to ten years.

Gun Owners of America is not going to wade into the marijuana debate, but the fact that such a large group of people are losing their constitutional rights without even seeing the inside of a courtroom leaves a lot of people scratching their heads.

After all, this is the same Justice Department that, in 2009, basically advised federal prosecutors to turn a blind eye to medical marijuana use.

And, in a bit of twisted irony, the ATF itself has helped thousands of guns to get into the hands of known and dangerous drug cartels in Mexico through their operation “Fast and Furious,” which has led GOA to call for anti-gun Attorney General Eric Holder’s resignation.

The Justice Department makes it perfectly clear in the letter what it expects from gun dealers and gun purchasers, even though the Department and ATF obviously consider themselves above the law—both at the federal and state levels. According to the agency:

“Any person who uses or is addicted to marijuana, regardless of whether his or her State has passed legislation authorizing marijuana use for medical purposes, is an unlawful user of or addicted to a controlled substance, and is prohibited by Federal law from possessing firearms or ammunition.”

Notice that persons need not be regular users of the prescribed drug; just the mere possession of a prescription card from a doctor is sufficient to trigger the gun ban. Firearms dealers are further instructed by the ATF:

“If you are aware that the potential transferee is in possession of a card authorizing the possession and use of marijuana under State law, then you have ‘reasonable cause to believe’ that the person is an unlawful user of a controlled substance.”

As far as the overbearing, overreaching federal government is concerned, there is no need for a finding in a court of law that the person is a criminal, and there is no need for a finding that a person is a danger to self or others.

In fact, the gun ban is not even limited to when a person is actually using the drug for medical purposes. If you have the prescription card, you can’t own a gun—period.

This is just one example of how the government uses various existing laws to sweep people onto its gun prohibition list. The same thing occurred over the past decade to more than one hundred thousand military veterans who were prohibited from owning guns because of ailments such as Post Traumatic Stress Disorder (PTSD)—in this instance, too, without the benefit of due process of law.

The fact that the ATF is acting against a state-sanctioned medical issue is also particularly troubling. The reason Gun Owners of America has opposed ObamaCare so vociferously is over the concern that medical records databases could be abused by the government.

How many other heretofore legally prescribed drugs could be frowned upon by federal agencies and subject masses of people to a lifetime gun ban?

If the ruling in the ATF letter is allowed to stand, we will have allowed the federal bureaucracy to arbitrarily carve out—with no input from the peoples’ representatives in Congress—an entire class of persons for whom civil rights may as well not exist.

And, to make matters worse, this letter looks like more proof that the ATF is doing all it can to draw attention away from its own “gunrunning” scandal and to avoid investigating and punishing official wrongdoing along the U.S.—Mexico border.

John Velleco (jvelleco@gunowners.org) is Director of Federal Affairs for Gun Owners of America, a grassroots lobbying organization with over 300,000 members nationwide. GOA spokesmen are available for comment.